When the air filter gets dirty it becomes restrictive to airflow passing through it, so your unit needs to work harder to "breath in." This puts stress on the small components like the capacitor and can cause them to run on a higher amperage, costing you more to run. Restrictive airflow can also cause your outside condenser coils to freeze over, creating a ice block over the coils and cause your HVAC unit to blow warm air in the summer. Dirty filters also allow more dust and crap past the filter and to build up on the indoor evaporator coil. This lowers the efficiency of the heating and cooling system, costing you more money and not making you as comfortable as a properly operating unit would.
We recommend using the cheap throw away filters because of the low airflow restriction. Your heat pump or furnace has to work a lot less to breath in and get the full capacity of air past the cooling coils. The only downside is that the cheap throw away filters do not catch as many airborne particles as the 1" pleated filter. If you are looking for the ideal filter that does both, capture airborne particles and doesn't put the heat pump or furnace under a lot of stress, look for the 4" HEPA filter.
So what filters are the best? We love the 4" HEPA filters because it filters the dust and allergens better than either of the other filters plus it allows maximum airflow to pass through it, not stressing the AC system. The HEPA filters are becoming more commonly found at your local Home Depot or Lowes and they only need to be changed every 3 months. The only downside is that they are more expensive than the 1" pleated filters, but not that much more. It's best to take the advice of thousands of energy auditors and heat pump contractors and make change to HEPA air filters.
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